Photographic process printers use a cathode ray tube (CRT) to project an image onto a photosensitive media which is being conveyed past the face of the CRT. A fiber optic bundle attached to the CRT conducts the image from the phosphor display area on the face of the CRT to the photosensitive media. After the image is projected onto the media, the media is photographically processed to develop the image.
In order for this image to be recorded in focus and properly positioned on the photosensitive media, the CRT must be accurately aligned to a fixed distance from the media. Prior art devices have placed the media in direct contact with the CRT. If the media is in contact with the fiber optic bundle, however, unwanted artifacts are printed, caused by coupling differences between high spots on either the media or the surface of the fiber optic bundle that cause contact between the media and surface, and low spots on either the media or the fiber optic bundle surface which prevent contact. Keeping the media at a fixed distance from the surface of the fiber optic bundle prevents these artifacts. The distance must be small, however, to preserve focus of the image on the media.
Others have spaced media away from the CRT, however, for different reasons. U.S. Pat. No. 3,818,131 issued Jun. 18, 1974 to Emmons, entitled "Fiber Optic Cathode Ray Tube with Anti-Static Discharge Means" placed transparent tape between the CRT and the media in order to prevent static discharge between the CRT and the media. This static discharge problem was particularly severe in Emmons device because the device used electrostatic copying. In Emmons device, the transparent tape covered the entire display area of the CRT, however, this could interfere with the transmission of light between the CRT and the media. Also, transparent tape may discolor over time, which would interfere with light transmission, and transparent material causes contact artifacts if contact is not complete. Therefore, it is desirable to have an air gap between the display area and the media, rather than a transparent material, to allow the maximum possible light to transfer.
U.S. Pat. No. 3,609,233 issued Sep. 28, 1971 to Nagao entitled "Electron Tube Facsimile Apparatus" provides a separation between the CRT and the media in order to allow the image being projected to the media to also reflect back to a photocell. This separation was provided by a transparent film that moved in unison with the media or transparent material attached to the light-emitting end of the fiber optics. This device has the same limitations as Emmons device in that the transparent material could interfere with light transmission.
It is thus apparent that there is a need in the art for an improved method or apparatus which allows precise spacing between a CRT and media. There is further need for such an apparatus that provide such spacing without covering the phosphor display area to allow the maximum transmission of light. A still further need in the art is for a system to keep the media against the spacer so that the media is kept at a constant distance from the media. The present invention meets these and other needs.